Bit like the indian 737 that evacuated when an F/A mistook the reflection of the beacon on the engine at night as a fire. Mistakes do happen.
It is easy with hindsight to pour scorn on others. But two crew in the cockpit and the cabin crew tell you the plane is on fire, would you immediately send your copilot to have a look? With no indication of fire I am sure you would ask the cabin to double check. But if the plane really is on fire as we all know you have little time to get it down. Sending your co-pilot for a walk round the cabin would definitely slow things down in such a situation.
This was not an ideal outcome but much better than the other way around.